Hello,
I have a:
Carrier Weathermaker 8000 model 58WAV070-12 s/n 4999A11255
Carrier Tech 2000 compressor.

For the past two days I've had these intermittent problems:

The AC fan and compressor run continuously cooling the house down to 69 degrees. The thermostat makes the appropriate noise as the temperature passes the set temperature but the AC fan and compressor continue to run. The fan switch isset on auto. Voltages at the thermostat and the board are Red>>Yellow 0V.Red>>Green 0V. Red>>White 24V.

If I trip the circuit breaker to the compressor, it turns off, but the furnace fan continues to run. Voltages at the thermostat and the board are Red>>Yellow0V. Red>>Green 24V. Red>>White 24V.

If I set the thermostat to heat, the compressor continues to run, the fan cycles off for 45 seconds then starts back up.

Continuity along the wire from the thermostat to the board appears to be intact. The board LED is on and continuous. All connections to the board and thermostat are tight. I tried a cheap replacement thermostat with the same results.

The furnace and compressor will function appropriately for short periods, but then reverts back to this behavior.

What’s next?

 

Turn the stat to "off position", Then check the voltage across C and Y on the control module in the furnace. If 0 volts check the contactor in the AC unit. Could be stuck.

 

0 volts between red and yellow indicates a cooling demand ( or short).

Does the AC turn off when you remove the red wire from the stat?

 

Thanks for the suggestion.

The contactor was replaced 2 years ago even though it had not failed. That time it was the capacitor. I thought that if it was the contactor the compressor would run continuously, but fan operation would be normal.

The voltages at the control board are:
Yellow>>Common 24V kill switch pressed in
Yellow>>common 0V. switch not pressed

It's a real mix of symptoms. I don't want to replace the control board unless I'm sure that's the problem.

 

Thanks for your reply

No, the AC runs independent of connections at the stat. That's unless it's functioning normally (intermittent) then everything works as it should.

 

If stuck contactor both compressor and fan will run continuously. 24 V across Yellow and Common indicates the stat is calling for cooling.

 

Thanks again,

So your reply would indicate a bad stat?

The compressor and the fan continue to run with the stat disconnected, and the fan runs continuously with the power cut to the compressor.

 

Are you talking about the indoor fan?

 

Yes the furnace fan is the fan that runs continuously even when power is cut to the compressor.

 

Oh OK, yes the blower will run when 24v across Y and C. There should be 2 wires on the Y terminal. One coming from the stat and one going outside to start the condenser. If you pull both those wires off the blower and AC should stop. If that's the case then problem is in your stat.

 

Great help thanks,

Results:
Both wires disconnected blower and compressor off.
Yellow stat wire disconnected, compressor wire connected compressor off, blower running
Compressor wire disconnected, yellow stat wire connected compressor off, blower running

Is that what you would expect with a bad stat?

 

Not the result I expected, check volts across G and common. Make sure stat is set to AUTO fan.

 

Okay,

Stat set to off, fan to auto.
Compressor running, blower running
Common>>Yellow 24V.
Common>>Green 24V.
Common>>compressor wire connected to yellow 0V.

 

Have you checked the battery in the stat? Sure sounds like the stat is bad. Whats the make and model number of the stat?

 

Sorry for the delay,

First thing I did was the easy stuff. New batteries, make sure all wire connections were tight (especially because of the intermittent function), code from controller board, etc. The stat is a Lux TX1500. I did a full reset and reprogrammed the stat.

Since our last communication I checked the contactor. Looks clean, no loose connections. 24V. present across the LV side with stat turned off. Switch moved quickly back and forth when one pole disconnected and reattached. I did not check the HV side as the compressor is running well and making the house cold.

Measured voltage again at the controller board.
Common>> green stat wire 24V. fan on auto stat off
Common>>Yellow stat wire 24V.
Common>>compressor wire 0V.
Common>>yellow, green, or white screw on board 0V.

The compressor and blower run even when the stat is disconnected.
Any suggestions would be appreciated for this puzzling situation!!

 

Sounds like the short is between the stat and the furnace.

Remove the stat, disconnect Y and G from the furnace and measure each wire to furnace C (common).

I suspect that you need to pull new stat wire.

 

Interesting thread; Houston204 appears to be on the right track; or it is not wired right...

He already said that he tried another low cost RM-TH with same results, so it wouldn't be the RM-TH.

I have not read everything in the thread so not trying to troubleshoot it...

 

Start. With a pic of the air handler terminal boArd. Then remove the thermostat wires at the air handler. Jump R to G at the air handler, the blower alone should start, nothing else. Then add a second jumper,R to Y. The condenser should start, no chattering either. If condenser won't start, measure the contractor coil voltage in the condenser. If you see lower than 22 volts, like 19 volts, make sure the transformer is wired for 208V and not 240V. If everything works ok, remove the stat connection on the wall, and reconnect the thermostat wire to the terminal board at the furnace. Now retest the same jumping sequence as before, this time you'll do the test jumping from the disconnected stat wires sticking out of the wall. You should get the same result.

 

Thanks for the suggestions:

I disconnected all stat wires everything turned off.

Jumper wire
Red>>Green blower turned on and turned off when wire was disconnected.

Red>>Yellow compressor started and within 5 seconds the blower started (no connection to green). I disconnected the jumper wire, and blower and compressor continued to run until I let the switch go then everything stayed off.

I was hoping for a bad(shorted) stat wire, but it's sounding more like the controller board.
What's the general opinion?
I did test the contactor outside, and it appears to be functioning normally. There hasn't been any recent wiring before the problem started. The heating and AC has worked without a problem for about 10 years.

 

Solution

After testing and retesting everything I replaced the control board. Unfortunately it did not correct the problem.

I tried the cheap thermostat again without success.

Due to the intermittent nature of the problem I decided to replace the Stat (We were due for a new one anyway) and the Stat wire all the way back to the blower. While I was doing that I discovered that during some fairly recent construction, that actually ended almost 8 months ago, someone had secured the stat wire with zip ties and some plastic anchors. This caused a small breech in the insulation and a short that intermittently affected only the AC side. The heat had worked properly over the winter.

I thought I should post the solution to our problem in the hopes it may help someone else.

I installed a Venstar Color Touch 8900 Thermostat that went in without a hitch and is amazing in it's programmability, look, and usage data that it provides. I'm very happy with that upgrade and that the AC works properly again!!!!

 

That is great news

Intermittent problems can be tough to resolve.
I am happy that you found the cause of this problem.

 

You called it Houston
Thanks